Weather-resistant polyacetal resin composition and molded articles therefrom

ABSTRACT

The weather and UV-light resistance of polyacetal resin compositions is improved by incorporating into polyacetal resin a stabilizing package comprised of at least one stabilizing agent selected from benzotriazoles, benzophenones, aromatic benzoates, cyanoacrylates, oxalanilides, and hindered amines, and an acylate resin selected from homopolymers or copolymers of acrylic or methacrylic acid, and ester derivatives thereof.

FIELD OF INVENTION

The present invention relates to polyacetal resin compositionsexhibiting improved resistance to weathering and UV-light degradation.In particular, the present invention is embodied in a UV-lightstabilized polyacetal resin composition having excellent weatherresistance which includes a stabilizing package comprised of aweather/light stabilizing agent and an acrylate resin. The presentinvention also relates to molded articles comprised of the presentpolyacetal resin composition.

BACKGROUND AND SUMMARY OF THE INVENTION

As is well known, polyacetal resins are conventionally employed invarious end-use applications as engineering plastics due to theirexcellent physical properties, such as mechanical and electricalproperties as well as their chemical and heat resistance properties.However, new end-use applications for polyacetal resins demand furtherproperty improvements.

For example, further improvement in the weather resistance of thepolyacetal resin is required in some situations. In particular,conventional polyacetal resin used to form exterior parts of automobilesand components of electric and office appliances sometimes exhibitdiscoloration, surface smoothness deterioration (resulting in areduction of the surface gloss), and/or crack formation on the surfaceof such parts when exposed to ambient conditions in their intendedenvironment of use (e.g. solar (UV) light, rain, fluorescent lamps,and/or air). These deleterious effects not only impair the appearance ofmolded parts, but also adversely cause deterioration of the parts'mechanical properties.

It has been proposed to incorporate various stabilizers in polyacetalresins so as to improve their weather and/or UV-light resistanceproperties and thus overcome the above-noted effects. However, althoughsome improved effects can be obtained by prior stabilization proposals,the surface conditions (e.g. cracks, reduced gloss, etc.) and themechanical properties of molded polyacetal articles after being exposedto solar rays (ultraviolet rays), rain, and like outdoor elements arenot always satisfactory. Thus further improvement in the weather/lightresistance of polyacetal resin is needed.

A mere increase in the amount of the stabilizing agent in order tosatisfy such a requirement only improves to a limited extent theweather/light resistance properties of polyacetal resin. In addition,increasing the amount of the stabilizing agent may impair the mechanicalproperties (such as tensile strength and bending strength of thepolyacetal resin). Furthermore, stabilizing agents tend to form molddeposits during molding thereby reducing the dimensional accuracy and/orimpairing the appearance of molded articles (not to mention theadditional time needed to clean the mold) thereby reducing theefficiency of the molding operation. Another problem with conventionalstabilizing agents is their tendency to "bleed" on the surfaces ofmolded articles thereby deleteriously affecting the articles' visualappearance.

According to the present invention, weather stabilized polyacetalmolding compositions are provided having a major amount of a polyacetalresin, and a weather stabilizing effective amount of a stabilizingsystem which includes (i) at least one weather/light stabilizing agent,and (ii) an acrylate resin.

More specifically, according to the present invention, aweather-resistant polyacetal resin composition is comprised of (A) apolyacetal resin, (B) between about 0.01 to about 5 wt. %, based on thetotal weight of the composition, of a stabilizing agent and (C) betweenabout 1 to 40 wt. %, based on the total weight of the composition, of anacrylate resin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polyacetal resin component (A) used in the present invention is apolymeric compound comprising oxymethylene groups (--CH₂ O--) as theprincipal units in the polymer chain. Any polyoxymethylene homoplymer,copolymer, terpolymer and/or block copolymer containing a small amountof other constituent units in addition to the oxymethylene groups maysatisfactorily be employed. The polyacetal resin component (a) may haveeither a linear structure, or a branched or crosslinked structure. Thedegree of polymerization of the polyacetal resin component (a) is notparticularly limited.

The preferred stabilizing agents usable as component (B) in the presentinvention, include one or more compounds selected from the groupconsisting of (1) benzotriazoles, (2) benzophenones, (3) aromaticbenzoates, (4) cyanoacrylates, (5) oxalanilides and (6) hindered amines.

Examples of such stabilizing agents include

(1) benzotriazoles:

2-(2'-hydroxy-5'-methylphenyl) benzotriazole,

2-(2'-hydroxy-3',5'-di-t-butylphenyl) benzotriazole,

2-(3,5-di-t-amyl-2-hydroxyphenyl) benzotriazole,

2-(2'-hydroxy-3',5'-diisoamylphenyl) benzoatariazole,

2-[2-hydroxy-3,5-bis(α,α-dimethylbenzyl)phenyl]benzotriazole and

2-(2'-hydroxy-4'-octoxyphenyl)benzotriazole

(2) benzophenones:

2,4-dihydroxybenzophenone,

2-hydroxy-4-methoxybenzophenone,

2-hydroxy-4-octoxybenzophenone,

2-hydroxy-4-dodecyloxybenzophenone,

2,2'-dimethoxybenzophenone,

2-hydroxy-4-methoxy-5-sulfobenzophenone and

2-hydroxy-4-oxybenzylbenzophenone,

(3) aromatic benzoates:

p-t-butylphenyl salicylate and p-octylphenyl salicylate,

(4) cyanoacrylates:

2-ethylhexyl,

2-cyano-3,3'-diphenyl-acrylate and ethyl 2-cyano-3,3'-diphenylacrylate,and

(5) oxalanilides:

N-(2-ethylphenyl)-N'-(2-ethoxy-5-t-butylphenyl)oxalamide and

N-(2-ethylphenyl)-N'-(2-ethoxyphenyl) oxalamide.

The hindered amines (6) usable herein are piperidine derivatives havinga sterically hindered group, such as

4-acetoxy-2,2,6,6-tetramethylpiperidine,

4-stearoyloxy-2,2,6,6-tetramethylpiperidine,

4-acryloyloxy-2,2,6,6-tetramethylpiperidine,

4-methoxy-2,2,6,6-tetramethylpiperidine,

4-benzoyloxy-2,2,6,6-tetramethylpiperidine,

4-cyclohexyloxy-2,2,6,6-tetramethylpiperidine,

4-phenoxy-2,2,6,6-tetramethylpiperidine,

4-benzyloxy-2,2,6,6-tetramethylpiperidine,

4-(phenylcarbamoyloxy)-2,2,6,6-tetramethylpiperidine,

bis(2,2,6,6-tetramethyl-4-piperidyl) oxalate,

bis(2,2,6,6-tetramethyl-4-piperidyl) malonate,

bis(2,2,6,6-tetramethyl-4-piperidyl) adipate,

bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate,

bis(1,2,2,6,6-pentamethylpiperidyl) sebacate,

bis(2,2,6,6-tetramethyl-4-piperidyl) terephthalate,

1,2-bis(2,2,6,6-tetramethyl-4-piperidyloxy) ethane,

bis(2,2,6,6-tetramethyl-4-piperidyl) hexamethylene-1,6-dicarbamate, bis(1-methyl-2,2,6,6-tetramethyl-4-piperidyl) adipate andtris(2,2,6,6-tetramethyl-4-piperidyl) benzene-1,3,5-tricarboxylate.

High molecular piperidine derivative polycondensates such as dimethylsuccinate/1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidinepolycondensate are also usable.

The above-noted stabilizing agent is used alone or preferably incombinations of two or more. Particularly, a combination of one or moreof the weather stabilizers (1) to (5) with a hindered amine (6) ispreferred.

The stabilized agent is preferably used in an amount of between about0.01 to about 5 wt. %, particularly about 0.02 to about 3 wt. %, basedon the total weight of the composition. When the stabilizing agent ispresent in insufficient amounts, its beneficial effect cannot beobtained. On the other hand, an excess amount of the stabilizing agentis economically disadvantageous, impairs the mechanical properties ofthe resin, and causes mold deposits.

The present invention is particularly characterized in that thestabilizing agent (B) and the acrylate resin component (C) areincorporated as a stabilizing package in the polyacetal resincomposition (A).

The acrylate resins usable in the present invention include homopolymersof acrylic acid, and its ester derivatives (such as methyl acrylate,ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate,n-hexyl acrylate and n-octyl acrylate), methacrylic acid and its esterderivatives (such as methyl methacrylate, ethyl methacrylate, n-propylmethacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutylmethacrylate, n-amyl methacrylate and n-octyl methacrylate); andcopolymers of acrylic acid methacrylate acid with a comonomer, such as,for example, styrene, acrylonitrile, butadiene or isoprene. Methylmethacrylate homopolymer and copolymers comprising methyl methacrylateas the main component and an acrylic ester, methacrylic ester (otherthan methyl methacrylate), acrylic acid, methacrylic acid, styrene oracrylonitrile are preferably used.

The amount of the acrylate resin component (C) used herein is preferablybetween about 1 to about 40 wt. %, particularly between about 3 to 20wt. %, based on the total composition weight. When the amount of theacrylate resin component (C) is insufficient, the beneficial weather andlight resistance effects (particularly prevention of crank formation onsurfaces of molded articles) cannot be obtained. On the other hand, whenthe amount the acrylate resin component (C) is excessive, a deleteriousinfluence on the mechanical properties and the thermal stability of theresin is obtained.

When the acrylate resin component (C) is used in combination with thestabilizing agent (B), the weather and UV-light properties of polyacetalresin are synergistically increased.

It is usually desirable to add other known stabilizers to thecomposition of the present invention to further increase its thermalstability, etc. For this purpose, known antioxidants, nitrogencompounds, and alkali and alkaline earth metal compounds are preferablyused either alone or in combinations of two or more such components.

The compositions of the present invention can be colored with a desiredcoloring agent by suitably adding carbon black, dye or pigment.

The combination of the stabilizing agent with the acrylate resinaccording to the present invention is not only effective in impartingimproved weather resistance to the resin material per se, but also isquite effective in preventing discoloration of the dyes and/or pigmentsadded to the resin. Thus, the present invention is quite effective inmaintaining the appearance and quality of colored polyacetal resinproducts.

The combination of the stabilizing agent and carbon black is alsoeffective in further improving the weather/light stability of thepolyacetal resin.

The compositions of the present invention may further contain otherknown additives conventionally used in engineering resins to obtaindesired properties. These optional additives include, for example,lubricants, nucleating agents, mold release agents, antistatic agents,surfactants, organic polymeric materials, and inorganic or organic,fibrous, granular or platy fillers. These additives can be used eitheralone or in combinations of two or more.

The compositions of the present invention can be produced by knownprocessing techniques using known apparatus ordinarily employed in theproduction of synthetic resin compositions. For example, the necessarycomponents can be premixed, with the mixture subsequently being kneadedand extruded via a single-screw or twin-screw extruder to form pellets.The pellets may then be molded to form the moldings. The preparation ofthe resin compositions can be performed simultaneously with the moldingof articles from the compositions by means of a molding machine.Alternatively, a portion, or the entirety of the resin components may bepulverized in order to facilitate their dispersion or mixing, with thecomponents subsequently being mixed and melt-extruded to form pellets.

The stabilizing agent and the other additives may be incorporated intothe resin in any step of the process. As a matter of course, however,they are usually added immediately before the production of moldedarticles, i.e. the final product.

The resin compositions of the present invention can be molded by anymolding process conventional in the plastics-forming art, for example,extrusion, injection, compression, vacuum, blow and foam moldingprocesses.

The compositions of the present invention comprising a polyacetal resincontaining a stabilizing agent and a specified acrylate resinincorporated therein exhibit significantly improved effects on theirsurface conditions (particularly significantly increased crack formationtime) in addition to exhibiting less reduction in tensile strength whenexposed to ultraviolet light or water for prolonged time periods ascompared to conventional acrylic resin-free polyacetal compositions.

Therefore, the weather-resistant polyacetal resin compositions of thepresent invention may suitably be used to form molded parts for end-useapplications demanding excellent weather/light resistance. Inparticular, the compositions of this .Iadd.invention can be.Iaddend.exposed to solar (UV) rays, rain and air for prolonged timeperiods, .Iadd.and formed into suitable parts .Iaddend.such as exteriorparts for automobiles (for example, outer door handles, fuel lids, wheelcovers, front fenders and spoilers), electric appliances, cameras, otherindustrial parts, construction materials, pipes, sundries and householdgoods.

The following nonlimiting Examples will further illustrate the presentinvention.

EXAMPLES

In the following Examples, the characteristic values, such as weatherresistance, were determined by the following methods:

(1) Crack formation time:

Test pieces were irradiated with ultraviolet rays with a weather meter(WBL-SUN-HCH; a product of Suga Shikenki Co., Ltd.) having a rain sprayat 63° C. The surfaces of the test pieces were observed with amagnifying lens at 10x magnification to confirm the crack formation. Thecrack formation time was the time at which crack formation began, andthus .Iadd.the .Iaddend.higher the time entry, the better the weatherresistance.

(2) Changes in surface conditions:

Test pieces were irradiated with ultraviolet rays with the same weathermeter as that described above for a predetermined period of time.Changes in the hue and gloss of the test pieces after irradiation wereobserved. The extent of the changes was classified into five groups,with the smaller numbers corresponding to lesser changes (i.e. lessergloss impairment and discoloration), and vice-versa.

(3) Tensile strength:

The tensile strengths of the test pieces were determined by conventional.[.methology.]. .Iadd.methodology .Iaddend.(ASTM-D 638) beforeirradiation with the light of the weather meter and after theirradiation for a predetermined period of time.

Examples 1 to 12 and Comparative Examples 1 to 11

A weather (light) stabilizer (B) and an acrylate resin (methylmethacrylate resin) (C) were added to a polyacetal resin (A) (tradename: Duracon M 90; a product of Polyplastics Co., Ltd.) as shown inTable 1 to obtain a mixture. The mixture was melt-kneaded with a 30 mmdouble-screw extruder to produce a composition in pellet form. Thepellets were then molded into test pieces with an injection moldingmachine. The weather resistance and other characteristics weredetermined as above described. For comparison, the same procedure asabove was repeated except that the weather (light) stabilizer (B) or theacrylate resin (C) were each used alone. The results appear in Table 1below.

                                      TABLE 1                                     __________________________________________________________________________                (B)                                                                           Weather     (C)       Crack         Tensile strength              (A)         (light      Acrylic   forma-        (kg/cm.sup.2)                 Polyacetal  stabilizer) resin                                                                              Carbon                                                                             tion                                                                              Surface condition                                                                       Before                                                                            after                     resin       Note 1                                                                              Note 2                                                                              Note 3                                                                             black                                                                              time                                                                              after irradiation                                                                       irradia-                                                                          irradiation for:          No.   (wt. %)                                                                             (wt. %)                                                                             (wt. %)                                                                             (wt. %)                                                                            (wt. %)                                                                            (b) 300 h                                                                            600 h                                                                            1000 h                                                                            tion                                                                              600                                                                              1000                                                                              2000               __________________________________________________________________________                                                               h                  Example                                                                       1     89.5  B-1 (0.5)                                                                           --    C-1 (10)                                                                           --   380 1  2  5   645 635                                                                              580 480                2     89.5  B-1 (0.5)                                                                           --    C-2 (10)                                                                           --   400 1  2  5   640 630                                                                              570 475                3     89.5  B-1 (0.5)                                                                           --    C-3 (10)                                                                           --   400 1  2  5   643 640                                                                              574 471                4     89.5  B-2 (0.5)                                                                           --    C-2 (10)                                                                           --   380 1  2  5   638 635                                                                              573 450                5     89.5  B-3 (0.5)                                                                           --    C-2 (10)                                                                           --   420 1  2  5   640 636                                                                              560 455                6     89.5  --    B-4 (0.5)                                                                           C-2 (10)                                                                           --   380 1  2  5   643 640                                                                              570 465                7     89.5  --    B-5 (0.5)                                                                           C-2 (10)                                                                           --   380 1  2  5   640 638                                                                              568 470                8     94.5  B-1 (0.5)                                                                           --    C-2 (5)                                                                            --   350 1  2  5   635 625                                                                              570 445                9     79.5  B-1 (0.5)                                                                           --    C-2 (20)                                                                           --   440 1  2  4   650 655                                                                              600 460                10    89.5  B-3 (0.25)                                                                          B-5 (0.25)                                                                          C-2 (10)                                                                           --   580 1  2  4   642 640                                                                              601 510                11    89.5  B-1 (0.25)                                                                          B-4 (0.25)                                                                          C-2 (10)                                                                           --   600 1  2  3   644 641                                                                              605 505                12    89.0  B-1 (0.25)                                                                          B-4 (0.25)                                                                          C-2 (10)                                                                           (0.5)                                                                              1150                                                                              1  1  2   648 650                                                                              640 613                Com-                                                                          parative                                                                      Example                                                                       1     99.5  B-1 (0.5)                                                                           --    --   --   280 2  3  5   625 610                                                                              520 350                2     99.5  B-2 (0.5)                                                                           --    --   --   260 3  4  5   628 600                                                                              510 360                3     99.5  B-3 (0.5)                                                                           --    --   --   300 2  3  5   628 595                                                                              508 340                4     99.5  --    B-4 (0.5)                                                                           --   --   240 3  4  5   630 605                                                                              522 338                5     99.5  --    B-5 (0.5)                                                                           --   --   280 2  3  5   626 590                                                                              513 340                6     99.5  B-3 (0.25)                                                                          B-5 (0.25)                                                                          --   --   380 1  2  5   630 620                                                                              550 420                7     99.5  B-1 (0.25)                                                                          B-4 (0.25)                                                                          --   --   400 1  2  5   627 619                                                                              540 410                8     99.0  B-1 (0.25)                                                                          B-4 (0.25)                                                                          --   (0.5)                                                                              880 1  2  4   633 635                                                                              610 560                9     90.0  --    --    C-1 (10)                                                                           --   100 4  5  5   650 600                                                                              483 immea-             10    90.0  --    --    C-2 (10)                                                                           --   140 4  5  5   648 603                                                                              500 sur-               11    90.0  --    --    C-3 (10)                                                                           --   160 4  4  5   645 598                                                                              470 able               __________________________________________________________________________     Note 1                                                                        B1: 2(3.5 di4-amyl-2-hydroxyphenyl)benzotriazole                              B2: 2[2hydroxy-3.5 bis(α,dimethylbenzyl)phenyl]benzotriazole            B3: 2hydroxy-4-oxybenzylbenzophenone                                          Note 2                                                                        B4: bis(2.2.6.6 tetramethyl4-piperidyl)sebacate                               B5: dimethyl succinate/1(2-hydroxyethyl)-4-hydroxy-2.2.6.6                    tetramethylpiperidine polycondensate                                          Note 3                                                                        C1: methyl methacrylate resin (average molecular weight: 200,000)             C2: methyl methacrylate resin (average molecular weight: 50,000)              C3: methyl methacrylate resin (average molecular weight: 7,000)          

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A weather stabilized polyacetal moldingcomposition comprising a blend of a major amount of a polyacetal resin,and a weather stabilizing effective amount of a stabilizing system whichconsists essentially of (i) between about .[.1 to about 40.]. .Iadd.0.01to about 5 .Iaddend.percent by weight, based upon the total weight ofthe composition of benzotriazoles, benzophenones, aromatic benzoates,cyanoacrylates, oxalanilides, and hindered amines, and (ii) betweenabout .[.3 to about 20.]. .Iadd.1 to about 40 .Iaddend.percent byweight, based on the total composition weight, of a homopolymer orcopolymer derived from acrylic acid or methacrylic acid, and esterderivatives thereof.
 2. A stabilized polyacetal molding composition asin claim 1, wherein said stabilizing system includes as component (ii) acopolymer of acrylic acid or methacrylic acid with at least onecomonomer selected from styrene, acrylonitrile, butadiene, and ioprene.3. A stabilized polyacetal molding composition as in claim 2, whereinsaid component (ii) is a homopolymer or copolymer of methylmethacrylate. .[.4. A stabilized composition as in claim 1, whereincomponent (i) is present in an amount between about 0.01 to about 5percent by weight based upon the total weight of the composition..]. 5.A stabilized composition as in claim 1, wherein component (i) is presentin an amount between about 0.02 to about 3 percent by weight based uponthe total weight of the composition.
 6. Molded articles comprising themolding composition of claim 1 or
 2. .Iadd.7. A stabilized polyacetalmolding composition as in claim 1, wherein component (ii) is present inan amount between about 3 to about 20 percent by weight, based upon thetotal weight of the composition. .Iaddend.